In various industries, various efforts to reduce environmental impacts have been made on a global scale. In particular, in the automobile industry, the development of techniques has progressed for the spread of not only a gasoline engine vehicle having superior fuel efficiency but also a so-called eco-car such as a hybrid vehicle or an electric vehicle and for further improvement in the performance of the vehicles.
Incidentally, in an exhaust system for exhaust gas that connects a vehicle engine and a muffler to each other, a catalytic converter for purifying exhaust gas is generally provided.
The engine may emit environmentally harmful materials such as CO, NOx, or unburned HC or VOC. In order to convert such harmful materials into environmentally acceptable materials, catalyst layers formed of a noble metal catalyst such as palladium or platinum are formed on cell wall surfaces of a substrate including plural cells. More specifically, on the cell wall surfaces of the plural cells, the catalyst layers are formed in a longitudinal direction of the substrate which is a direction in which exhaust gas flows. By causing exhaust gas to flow through the catalytic converter including the substrate having the above-described configuration, CO is converted into CO2, NOx is converted into N2 and O2, and VOC is burned to produce CO2 and H2O.
However, for example, for a substrate including cells having a honeycomb structure, a catalytic converter having a uniform cell density of the substrate is generally used. However, since an exhaust gas flow rate distribution in a center region of a cross-section of the substrate is higher than that in a peripheral region thereof, there is a problem in that the catalyst layers of the entire substrate cannot be sufficiently utilized. Therefore, by using a catalytic converter in which a cell density of a center region of a substrate is higher than that of a peripheral region thereof in consideration of the above exhaust gas flow rate distribution, a difference between flow rate distributions in a cross-section of a substrate can be reduced as much as possible. Accordingly, catalyst layers of the entire catalytic converter can be efficiently utilized to purify exhaust gas.
Here, PTL1 discloses a technique of improving exhaust gas purification performance by causing the amount of a noble metal catalyst supported on a center region (here, middle portion) to be different from that supported on a peripheral region (here, peripheral portion), for example, in a substrate having a honeycomb structure in which the cell density is uniform in the entire catalytic converter (here, catalyst body). More specifically, in the catalyst body, the amount of catalyst per unit volume supported on the middle portion having a large amount of gas flow is set to be 1.1 times or higher than that on the peripheral portion.
By varying the cell density in a cross-section of a substrate as described above, pressure loss can be reduced, and exhaust gas purification performance can be improved.
However, when a catalytic converter having different cell densities in a cross-section of a substrate as described above is formed, the apparent catalyst coating amount may sometimes vary between a region having a high cell density and a region having a low cell density. This is because, even when the amounts of catalyst are the same, the thickness of a catalyst layer supported on one cell may vary. More specifically, this is because the region having a high cell density has a larger geometric surface area (GSA) of a honeycomb structure than that of the region having a low cell density and thus has a small thickness of a catalyst layer. At this time, in the region having a low cell density, the thickness of a catalyst layer is relatively large. In this case, the gas diffusibility to a deep portion of the catalyst layer deteriorates. Therefore, in the region having a low cell density, sufficient exhaust gas purification performance cannot be expected. This issue is not mentioned in the description of the catalytic converter disclosed in PTL 1 and cannot be solved by this catalytic converter.